I am trying to create a class in python that overrides a (pure) virtual function in a C++ class (using boost.python). The catch is that the C++ class is created via a static member function (all constructors are private or deleted). I have successfully created the class Base and a BaseWrap class that python "knows" about. I have also been able to create a pure virtual function that can be overridden in python. However, my problem is when a member function of Base calls the pure virtual function. When this happens, the class cannot find the python implementation and the program crashes.
Here is the C++ code:
#include <iostream>
#include <boost/python.hpp>
#include <boost/static_assert.hpp>
#define CREATE(NAME) \
static std::shared_ptr<NAME> Create() { \
std::cout << "STATIC BASE CREATE" << std::endl; \
return std::make_shared<NAME>(); \
}
class Base {
protected:
Base() { std::cout << "BASE DEFAULT CONSTRUCTOR" << std::endl; }
private:
std::string CallSay() {
return Say();
}
virtual std::string Say() const = 0;
};
class BaseWrap : public Base, public boost::python::wrapper<Base> {
public:
BaseWrap() : Base() { std::cout << "BASEWRAP DEFAULT CONSTRUCTOR" << std::endl; }
virtual std::string Say() const override
{
std::cout << "C++ Say" << std::endl;
return this->get_override("say") ();
}
CREATE(BaseWrap)
};
BOOST_PYTHON_MODULE(Example)
{
namespace python = boost::python;
// Expose Base.
python::class_<BaseWrap, std::shared_ptr<BaseWrap>, boost::noncopyable>("Base", python::no_init)
.def("__init__", python::make_constructor(&BaseWrap::Create))
.def("Say", python::pure_virtual(&Base::Say))
.def("CallSay", &Base::CallSay);
}
and the python code to test the issue:
import sys
import Example
class PythonDerived(Example.Base):
def __init__(self):
print "PYTHON DEFAULT CONSTRUCTOR"
Example.Base.__init__(self)
def Say(self):
return "Python Say"
d = PythonDerived()
print d
print
print d.Say()
print
print d.CallSay()
Which, when run, gives the output:
PYTHON DEFAULT CONSTRUCTOR
STATIC BASE CREATE
BASE DEFAULT CONSTRUCTOR
BASEWRAP DEFAULT CONSTRUCTOR
<__main__.PythonDerived object at 0x1091caf70>
Python Say
C++ Say
Traceback (most recent call last):
File "test.py", line 20, in <module>
print d.CallSay()
TypeError: 'NoneType' object is not callable
It looks like the Base::CallSay method is finding the implementation of BaseWrap::Say but cannot find the python implementation. Does anyone know why or how make this work?
Thanks!
This looks as though it is a bug in Boost.Python.
The boost::python::wrapper hierarchy is not getting initialized in the functor returned from boost::python::make_constructor. With the wrapper hierarchy having no handle to the Python object, get_override() returns NoneType, and attempting to call NoneType raises the TypeError exception.
To resolve this, one can explicitly initialize the wrapper hierarchy. Below is a complete example that provides a generic way to accomplish this. Instead of using make_constructor(), one can use make_wrapper_constructor(). I have opted to not use C++11 features. As such, there will be some boilerplate code that could be reduced with variadic templates, but porting to C++11 should be fairly trivial.
#include <iostream>
#include <boost/function_types/components.hpp>
#include <boost/function_types/result_type.hpp>
#include <boost/make_shared.hpp>
#include <boost/mpl/insert.hpp>
#include <boost/python.hpp>
namespace detail {
/// @brief wrapper_constructor will force the initialization
/// of the wrapper hierarchy when a class is held by
/// another type and inherits from boost::python::wrapper.
template <typename Fn>
class wrapper_constructor
{
public:
typedef typename boost::function_types::result_type<Fn>::type result_type;
public:
/// @brief Constructor.
wrapper_constructor(Fn fn)
: constructor_(boost::python::make_constructor(fn))
{}
/// @brief Construct and initialize python object.
result_type operator()(boost::python::object self)
{
constructor_(self);
return initialize(self);
}
/// @brief Construct and initialize python object.
template <typename A1>
result_type operator()(boost::python::object self, A1 a1)
{
constructor_(self, a1);
return initialize(self);
}
// ... overloads for arguments, or use variadic templates.
private:
/// @brief Explicitly initialize the wrapper.
static result_type initialize(boost::python::object self)
{
// Extract holder from self.
result_type ptr = boost::python::extract<result_type>(self);
// Explicitly initialize the boost::python::wrapper hierarchy.
initialize_wrapper(self.ptr(), // PyObject.
get_pointer(ptr)); // wrapper hierarchy.
return ptr;
}
private:
boost::python::object constructor_;
};
} // namespace detail
/// @brief Makes a wrapper constructor (constructor that works with
/// classes inheriting from boost::python::wrapper).
template <typename Fn>
boost::python::object make_wrapper_constructor(Fn fn)
{
// Python constructors take the instance/self argument as the first
// argument. Thus, inject the 'self' argument into the provided
// constructor function type.
typedef typename boost::function_types::components<Fn>::type
components_type;
typedef typename boost::mpl::begin<components_type>::type begin;
typedef typename boost::mpl::next<begin>::type self_pos;
typedef typename boost::mpl::insert<
components_type, self_pos, boost::python::object>::type signature_type;
// Create a callable python object that defers to the wrapper_constructor.
return boost::python::make_function(
detail::wrapper_constructor<Fn>(fn),
boost::python::default_call_policies(),
signature_type());
}
class Base
{
protected:
Base(int x) : x(x) { std::cout << "BASE DEFAULT CONSTRUCTOR" << std::endl; }
virtual ~Base() {}
int x;
public:
std::string CallSay() { return Say(); }
virtual std::string Say() const = 0;
};
class BaseWrap:
public Base,
public boost::python::wrapper<Base>
{
public:
BaseWrap(int x):
Base(x)
{ std::cout << "BASEWRAP DEFAULT CONSTRUCTOR" << std::endl; }
virtual std::string Say() const
{
std::cout << "C++ Say: " << x << std::endl;
return this->get_override("Say")();
}
static boost::shared_ptr<BaseWrap> Create(int x)
{
return boost::make_shared<BaseWrap>(x);
}
};
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
// Expose Base.
python::class_<BaseWrap, boost::shared_ptr<BaseWrap>,
boost::noncopyable>("Base", python::no_init)
.def("__init__", make_wrapper_constructor(&BaseWrap::Create))
.def("Say", python::pure_virtual(&Base::Say))
.def("CallSay", &Base::CallSay)
;
}
And its usage:
>>> import example
>>> class PythonDerived(example.Base):
... def __init__(self, x):
... print "PYTHON DEFAULT CONSTRUCTOR"
... example.Base.__init__(self, x)
... def Say(self):
... return "Python Say"
...
>>> d = PythonDerived(5)
PYTHON DEFAULT CONSTRUCTOR
BASE DEFAULT CONSTRUCTOR
BASEWRAP DEFAULT CONSTRUCTOR
>>> d
<__main__.PythonDerived object at 0xb7e688ec>
>>> d.Say()
'Python Say'
>>> d.CallSay()
C++ Say: 5
'Python Say'